The present invention relates to a torque distribution control apparatus for a four wheel drive.
In general, there has been known a torque distribution control apparatus for a four wheel drive which drives four wheels by engine outputs, wherein the torque distribution for each wheel is not always equivalent but is variably controlled so as to optimize operating conditions.
Japanese Unexamined Patent Publication No. 1-247221 has disclosed a torque distribution change system wherein the driving force of each wheel is absorbed by a brake correspondingly to load movement, which occurs according to the motion of a car such as acceleration, turning and the like, in order to give the driving force corresponding to the load movement for each wheel, because the load movement causes the driving force allowed by the wheels to be nonuniform.
Referring to the torque distribution change system, the car operates almost correspondingly to the change of a steering angle and a car speed on a road of which friction coefficient (hereinafter referred to as .mu. if necessary) is high. Accordingly, there are not caused problems. On a low .mu. road, however, a slip rate of a tire is great. Consequently, the motion of the car is delayed with the change of the steering angle and car speed. By way of example, even if steering is completed, the lateral load movement remains.
Accordingly, in the case where torque distribution control is to be executed between right and left wheels by detecting an actual lateral acceleration of the car, i.e., based on the lateral load movement, torque distribution is executed by the lateral acceleration which remains on the car even if the steering is completed. In that case, excessive autorotation moment occurs on the car owing to the driving torque which is laterally ununiform. Consequently, the turning of the car is not normally carried out, i.e., straightness is lowered.
Japanese Unexamined Patent Publication No. 1-247223 has disclosed that the turning movement of the car is classified under three parts at the time of turning start, during constant turning and at the time of turning escape, so that the torque distribution control is executed correspondingly to the turning state of the car on the basis of a steering angle and a change rate thereof for front wheels. In other words, the brake absorbs the driving force of the front wheels to decrease the torque distribution for the front wheels such that the small turn of the car can be enhanced at the time of turning start. On the other hand, the brake absorbs the driving force of the rear wheels to decrease the torque distribution for the rear wheels such that the straightness can be enhanced at the time of turning escape. Consequently, the burden of the front and rear tires can become uniform.
In the above-mentioned case, when the steering angle is almost equal to 0, turning escape control is completed. Also at that time, the comparatively great lateral acceleration remains on the car on the low .mu. road. When the turning escape control is completed, the driving torque is increased so that the lateral slide (side skid) of the rear wheels is made greater. Consequently, the running stability of the car is damaged. In other words, the turning escape cannot be carried out quickly.
In the case where the next turning is carried out immediately after the turning escape is completed as in S-shaped running, there is executed the torque distribution control at the time of turning start. Consequently, the driving torque is further increased even if the lateral slide is caused on the rear wheels. Thus, the lateral slide is further increased.